3. III
FOREWORD
The National Curriculum Framework (NCF) 2005, recommends that children’s life at school
must be linked to their life outside the school. This principle marks a departure from the
legacy of bookish learning which continues to shape our system and causes a gap between
the school, home and community. The syllabi and textbooks developed on the basis of NCF
signify an attempt to implement this basic idea. They also attempt to discourage rote learning
and the maintenance of sharp boundaries between different subject areas. We hope these
measures will take us significantly further in the direction of a child-centred system of
education outlined in the National Policy on Education (1986).
The success of this effort depends on the steps that school principals and teachers will
take to encourage children to reflect on their own learning and to pursue imaginative activities
and questions. We must recognise that, given space, time and freedom, children generate
new knowledge by engaging with the information passed on to them by adults. Treating the
prescribed textbook as the sole basis of examination is one of the key reasons why other
resources and sites of learning are ignored. Inculcating creativity and initiative is possible
if we perceive and treat children as participants in learning, not as receivers of a fixed body
of knowledge.
These aims imply considerable change in school routines and mode of functioning.
Flexibility in the daily time-table is as necessary as rigour in implementing the annual
calendar so that the required number of teaching days are actually devoted to teaching. The
methods used for teaching and evaluation will also determine how effective this textbook
proves for making children’s life at school a happy experience, rather than a source of
stress or boredom. Syllabus designers have tried to address the problem of curricular burden
by restructuring and reorienting knowledge at different stages with greater consideration
for child psychology and the time available for teaching. The textbook attempts to enhance
this endeavour by giving higher priority and space to opportunities for contemplation and
wondering, discussion in small groups, and activities requiring hands-on experience.
The National Council of Educational Research and Training (NCERT) appreciates the
hard work done by the textbook development committee responsible for this book. We
wish to thank the Chairperson of the advisory group in science and mathematics,
Professor J.V. Narlikar and the Chief Advisor for this book, Professor K. Muralidhar,
Department of Zoology, University of Delhi, Delhi for guiding the work of this committee.
Several teachers contributed to the development of this textbook. We are grateful to their
principals for making this possible. We are indebted to the institutions and organisations
which have generously permitted us to draw upon their resources, material and personnel.
We are especially grateful to the members of the National Monitoring Committee, appointed
2020-21
4. IV
by the Department of Secondary and Higher Education, Ministry of Human Resource
Development under the Chairmanship of Professor Mrinal Miri and Professor G.P. Deshpande,
for their valuable time and contribution.
As an organisation committed to systemic reform and continuous improvement in the
quality of its products, NCERT welcomes comments and suggestions which will enable us to
undertake further revision and refinement.
Director
New Delhi National Council of Educational
20 November 2006 Research and Training
2020-21
5. V
Biology is the study of life in its entirety. The growth of biology as a natural science during
the last 1000 years is interesting from many points of view. One feature of this growth is
changing emphasis. Initially it was description of life forms. Identification, nomenclature,
classification of all recorded living forms enjoyed the attention of scientists for a long time.
Description of their habitats and (in the case of animals) their behaviour was included in
this study. In later years, the focus was physiology and internal morphology or anatomy.
Darwinian ideas of evolution by natural selection changed the perception completely. Classical
descriptive and clueless biology found a theoretical framework in the evolutionary theory of
Darwin.
In the nineteenth and twentieth centuries, Physics and Chemistry were applied to Biology
and the new science of Biochemistry soon became the dominant face of biology. On one hand
Biochemistry was integrating with Physiology, becoming almost synonymous with it. On the
other hand it gave rise to Structural Biology (structure of biomacromolecules), originally called
Molecular Biology. The work of Bernal, Pauling, Watson and Crick, Hodgkins, Perutz and Kendrew,
Delbruck, Luria, Monod, Beadle and Tatum, Lederberg, Brenner, Benzer, Nirenberg, Khorana,
Mclintock, Sanger, Cohen, Boyer, Kornbergs (father and son), Leder, Chambon and scores of
others brought in and established a modern version of Molecular Biology dealing with life
processes at molecular level.
Physics and Chemistry dominated public perception of science for a long time.
Daytoday life of man was influenced by developments in Physics, Chemistry and their respective
manufacturing industries. Slowly and steadily, Biology, not to be left behind, demonstrated
its utility for human welfare. Medical practice, especially diagnostics, green revolution and
the newly emerging biotechnology and its success stories made the presence of biology felt
by the common man. Patent laws brought biology into political domain and commercial value
of biology became obvious.
For more than a century, classical and so-called reductionist biology fought artifical
battles. The fact is both are important. Ecology brought in synthesis of both approaches
and emphasised integrated understanding of biology. Form and process are both equally
important. Systems biology, using mathematical tools, is bringing about a modern synthesis
of both the aspects of Biology.
The Class XI and XII textbooks in biology essentially were to reflect these threads of
biological thought. While the Class XI book dealt with morphology, taxonomy, molecular and
cellular aspects of physiology, the Class XII book deals with the physiological process of
reproduction in flowering plants and humans, the principles of inheritance, the nature of
genetic material and its function, the contributions of biology to human welfare, basic
principles of biotechnological processes and their applications and achievements. The
Class XII book also relates genes to evolution on one hand and presents ecological interactions,
behaviour of populations and ecosystems on the other. Most important, the guidelines under
NCF-2005 have been followed in letter and spirit. The total learning load has been reduced
PREFACE
2020-21
6. VI
considerably and themes like environmental issues, adolescent problems and reproductive
health have been dealt with in some detail. Studied together, the class XI and class XII
textbooks in Biology would enable the student to —
(i) become familiar with the diversity of biological material.
(ii) appreciate and believe in the Darwinian evolutionary process exhibited by the living
world.
(iii) understand the dynamic state of constituents of living bodies, i.e., metabolic
basis of all physiological processes in plants, animals and microbes.
(iv) realise the structure and function of genetic material in directing the inherited
phenotype pattern as well as a mediator of evolutionary process.
(v) appreciate the profound contributions of biology to human welfare.
(vi) reflect on the physico-chemical basis of living processes and at the same time
realise the limitation of reductionism in understanding behaviour of organisms.
(vii) experience the humbling effect of this realisation that all living organisms are
related to each other by virtue of shared genetic material.
(viii) realise that biology is the story of the struggle of living organisms for existence
and survival.
One may notice a perceptible change in the writing style. Most of the chapters are written
in an easy dialogue style engaging the student constantly while some chapters are in the
form of critical comments on the subject matter. A number of questions have been provided
at the end of each chapter though answers to some may not be found in the text. Students
have to read supplementary material, upon advise from the teacher, to answer such questions.
I am thankful to Professor Krishna Kumar, Director NCERT; Professor G. Ravindra, Joint
Director, NCERT and Professor Hukum Singh, Head, DESM, NCERT for constant support. I
must place on record my deep appreciation for Dr B.K. Tripathi, Reader, DESM, NCERT for
his relentless efforts as coordinator in bringing out the Biology textbook for both the
Class XI and XII. All the members of the development team, the experts and reviewers, and
the school teachers have contributed enormously in the preparation of this book. I thank
them all. I am indeed highly thankful to the members of monitoring committee constituted
by Ministry of Human Resource Development for their valuable observation that helped in
the improvement of the book at the final stage. The book is prepared keeping in mind the
guidelines of the NCF-2005 especially the emphasis on reducing the learning load. We hope that
the book would meet the expectations of all the stakeholders. All suggestions for further
improvement are always welcome.
K. MURALIDHAR
Department of Zoology Chief Advisor
University of Delhi Biology Textbook for Class XII
2020-21
7. VII
TEXTBOOK DEVELOPMENT COMMITTEE
CHAIRPERSON, ADVISORY GROUP FOR TEXTBOOKS IN SCIENCE AND MATHEMATICS
J.V. Narlikar, Emeritus Professor, Inter University Centre for Astronomy and Astrophysics (IUCAA),
Pune University, Pune
CHIEF ADVISOR
K. Muralidhar, Professor, Department of Zoology, University of Delhi, Delhi
MEMBERS
Ajit Kumar Kavathekar, Reader (Botany), Sri Venkateswara College, University of Delhi, Delhi
B.B.P. Gupta, Professor, Department of Zoology, North-Eastern Hill University, Shillong
B.N. Pandey, Principal, Ordinance Factory Higher Secondary School, Dehradun
C.V.Shimray,Lecturer,Department ofEducationinScienceandMathematics,NCERT, New Delhi
DineshKumar,Reader,DepartmentofEducationinScienceandMathematics,NCERT, NewDelhi
J.P. Gaur, Professor, Department of Botany, Banaras Hindu University, Varanasi
J.S. Virdi, Reader, Department of Microbiology, University of Delhi, South Campus, New Delhi
K. Sarath Chandran, Reader (Zoology), Sri Venkateswara College, University of Delhi, Delhi
L.C. Rai, Professor, Department of Botany, Banaras Hindu University, Varanasi
M.M. Chaturvedi, Professor, Department of Zoology, University of Delhi, Delhi
N.V.S.R.K. Prasad, Reader (Botany), Sri Venkateswara College, University of Delhi, Delhi
Sangeeta Sharma, PGT (Biology), Kendriya Vidyalaya, JNU, New Delhi
Savithri Singh, Principal, Acharya Narendra Dev College, University of Delhi, Delhi
Shanti Chandrashekaran, Principal Scientist, Division of Genetics, I.A.R.I., New Delhi
Shardendu, Reader, Department of Botany, Science College, Patna University, Patna
Simminder K. Thukral, Assistant Professor, NIIT Institute of Information Technology, New Delhi
Sunaina Sharma, Lecturer (Biology), Rajkiya Pratibha Vikas Vidyalaya, Dwarka, New Delhi
T.R. Rao, Professor (Retd.) School of Enviornmental Studies, University of Delhi, Delhi
V.K. Kakaria, Reader, Regional Institute of Education, Bhopal
V.V. Anand, Reader, Regional Institute of Education, Mysore
MEMBER-COORDINATOR
B.K.Tripathi,Reader,Department ofEducationinScienceandMathematics,NCERT, NewDelhi
2020-21
8. National Council of Educational Research and Training (NCERT) gratefully acknowledges
the valuable contribution of K.R. Shivanna, Professor (Retd.), Department of Botany,
University of Delhi, Delhi; S.K. Saidapur, Professor, Department of Zoology, Karnataka
University Dharwad; Vani Brahmachari, Professor, Ambedkar Centre for Biomedical
Research, University of Delhi, Delhi; A.N. Lahiri Majumdar, Professor, Bose Institute, Kolkata;
Anil Tripathi, Professor, Department of Biotechnology, Banaras Hindu University, Varanasi;
J.L. Jain, Senior Physician, WUS Health Centre, University of Delhi, Delhi, in the development
of the Biology textbook for Class XII. NCERT is also grateful to K.R. Shivanna and
T.Subramanyam, IIT, Kanpur for some of the photographs used in the book.
NCERT sincerely acknowledges the contributions of the members who participated in the
review of the manuscripts – A.S. Dixit, Reader, Department of Zoology, North-Eastern Hill
University, Shillong; S.L. Varte, Lecturer, Department of Education in Science and Mathematics,
NCERT, New Delhi; Sushma Jairath, Reader, Department of Women’s Education, NCERT,
New Delhi; Mona Yadav, Lecturer, Department of Women’s Education, NCERT, New Delhi;
Poonam A. Kant, Reader (Zoology), Acharya Narendra Dev College, New Delhi; Mrs. Suvarna
Fonseca è Antao, Gr. I Teacher (Biology), Carmel Higher Secondary School, Nuvem, Goa;
Rashmi Mishra, PGT (Biology), Carmel Convent Senior Secondary School, BHEL, Bhopal;
Ishwant Kaur, PGT (Biology), D.M. School, RIE, Bhopal; A.K. Singh, PGT (Biology), Kendriya
Vidyalaya, Cantt, Varanasi; R.P. Singh, Lecturer (Biology), Rajkiya Pratibha Vikash Vidyalaya,
Kishan ganj, Delhi; M.K. Tiwari, PGT (Biology), Kendriya Vidyalaya, Mandsaur, Madhya Pradesh;
A.K. Ganguly, PGT (Biology), Jawahar Navodaya Vidyalaya, Roshnabad, Haridwar; Chaitali
Dixit, PGT (Biology), St. Anthony’s Higher Secondary School (Don Bosco), Shillong and Abhishek
Chari, Acharya Narendra Dev College, New Delhi.
Special thanks are due to Rita Sharma, Retd. Professor, RIE Bhopal, A.K. Mohapatra
Professor, RIE Bhubaneswar, J.S. Gill, Retd. Professor, DESM, NIE, G.V. Gopal, Professor, RIE
Mysore, Jaydeep Mandal, Professor, RIE Bhopal, C. Padmija, Professor, RIE Mysore,
Dr. Pushplata Verma, Associate Professor, DESM, NIE, Ishwant Kaur, Vice Principal, DM School
Ajmer for their valuable contribution in review and updation of the textbook.
The Council is highly thankful to Hukum Singh, Professor and Head, Department of
Education in Science and Mathematics, NCERT for his valuable support throughout the
making of this book.
The contributions of Deepak Kapoor, Incharge, Computer Station; Seema Mehmi and
Arvind Sharma, DTP operators; Deepti Sharma, Copy Editor; Rachna Dogra and Abhimanu
Mohanty, Proof Readers and APC office and administrative staff of Department of Education
in Science and Mathematics, NCERT also acknowledged.
The efforts of the Publication Department, NCERT, in bringing out this publication are
highly appreciated.
ACKNOWLEDGEMENTS
2020-21
9. CONTENTS
FOREWORD iii
PREFACE v
UNIT VI
REPRODUCTION 1-66
Chapter 1 : Reproduction in Organisms 3
Chapter 2 : Sexual Reproduction in Flowering Plants 19
Chapter 3 : Human Reproduction 42
Chapter 4 : Reproductive Health 57
UNIT VII
GENETICS AND EVOLUTION 67-142
Chapter 5 : Principles of Inheritance and Variation 69
Chapter 6 : Molecular Basis of Inheritance 95
Chapter 7 : Evolution 126
UNIT VIII
BIOLOGY IN HUMAN WELFARE 143-190
Chapter 8 : Human Health and Disease 145
Chapter 9 : Strategies for Enhancement in 165
Food Production
Chapter 10 : Microbes in Human Welfare 179
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10. X
UNIT IX
BIOTECHNOLOGY 191-216
Chapter 11 : Biotechnology : Principles and Processes 193
Chapter 12 : Biotechnology and its Applications 207
UNIT X
ECOLOGY 217-286
Chapter 13 : Organisms and Populations 219
Chapter 14 : Ecosystem 241
Chapter 15 : Biodiversity and Conservation 258
Chapter 16 : Environmental Issues 270
2020-21
